Fracture pressure models are used to create pre-drill fracture pressure–depth plots that are essential to the design of drilling and casing programmes in oil and gas wells, and for the future development of carbon capture and underground storage. Some of these models include an empirical term (stress ratio) that relates effective horizontal stress to effective vertical stress. Based on the literature, the stress ratio is assumed to vary with compaction and can be calculated from Poisson's ratio (). An alternative to models that utilize the stress ratio term is a model that relates fracture pressure to a constant fraction of the vertical stress. This paper demonstrates that a constant fraction of vertical stress is equivalent to a stress ratio that increases slightly with depth.
Estimation of how the changes with compaction is complicated by the multiple methods that are commonly employed to measure . The available static and dynamic data suggest a substantial decrease in with porosity reduction. Evaluation of in situ data from leak-off tests (LOTs) and associated fluid pressure data in Tertiary basins indicates that in situ does not decline as rapidly as indicated by the other methods of determination. In situ data indicate that calibration of the stress ratio from traditional methods is not appropriate and may substantially underestimate the fracture pressure (FP).
Fracture pressure models that utilize a constant fraction of vertical stress or estimate the stress ratio empirically from LOT data should be employed for pre-drill estimation of fracture pressure.
Thematic collection: This article is part of the Geopressure collection available at: https://www.lyellcollection.org/cc/geopressure